Minggu, 10 Juli 2011

Photoshop Tricks; Remove colour casts

Don’t let annoying colour casts blight what is an otherwise excellent image.  Foil their evil plans by bringing the Levels palette into play.

One advantage of living in  a world where the light is  always sunny and bright  is that colour casts would  be practically non-existent.  But an album full of photos taken in the  sun would make for a very boring photo  collection indeed, so there’s something to  be said for risking an odd colour cast here  and there.

Colour casts occur when one of the  RGB colours is stronger than the others  and usually happens when photos are  taken under artificial light. With fluorescent  lights this is often green and with tungsten  lighting it will be yellow. If the flash is  involved, you’re looking at a blue cast.  However, as this example shows, you can  also suffer from colour casts even if the  photo is taken outside. Natural daylight is  far from consistent and changes depending on location and time of day. For example, photos taken early in the morning will  appear more ‘bluish’ than those taken at midday. As the day ends, colour will become redder and warmer. But in addition to the natural light, you have to think about location. Photos taken under a canopy of trees, for example, can suffer from a green colour cast.

Photoshop provides the perfect tools for  removing these colour casts and bringing  back your neutral, natural colours. We’re going to use the Levels palette here to restore this photo to its rightful glory, by neutralising and defining the black, white and middle grey points. Use this tutorial on colour faded or old pictures; you’ll be amazed what it can do.

01 First things first  Copy the ‘before.tif’ file from the CD onto your desktop. Open it up Photoshop and take a good look where the problem areas are situated. The most obvious problem is the white in the car that has a yellow, greenish cast. Duplicate the layer by going to Layer>Duplicate Layer. Name the first layer Original and call the duplicate Retouch.
02 Prepare the workspace  Pick the Eyedropper tool and set the Sample size to 3 x 3 Average. Bring up the Info palette and go to View>Fit On Screen. Now we’re ready for optimising the image
03 Levels  In the Image>Adjustment menu, bring up the Levels palette. In this dialog box, beneath the Auto button are three eyedropper tools; these are used to set the black, grey and white points of the image. Double-click on the Eyedropper at the left to set the black point..
04 Setting the back point  After clicking on the left Eyedropper tool, the colour picker box will appear. Click in what seems the darkest, most black part of the image and look at the RGB Values. Here we have: R: 4, G:9 and B: 2. Set all the values to ‘4’ and click OK to conform. Click with the Eyedropper once more in the darkest area.
05 Set the white point  We will do exact the same thing for the white neutral point. Double-click on the right Eyedropper tool to bring up the colour picker. Set the eyedropper on a white part that has colour cast. Set all of the RGB values to 246. Click OK and click with the white loaded Eyedropper inside the image. You’ll notice that the cast disappears.
06 More white  The white in an image is one of the most important values; we don’t want to make it too white. The minimal white shouldn’t be less than 5% colour and the black shouldn’t be more than 95%. Check with the Eyedropper tool and see what values crop up in the Info palette.
07 Hue/Saturation Go to the Select menu and choose Color Range. Select the white of the car with the Eyedropper and set the Fuzziness to 95. Bring up the Hue/Saturation box and set the Saturation to -14 and decrease the Lightness to -3. This will remove the last bit of colour cast from your image.
08 Sharpen  Click out of the selection from the Color Range and examine your image. An Unsharp Mask on top of this would be useful. Go to the Filter menu and choose Unsharp Mask in the Sharpen menu. Set the Amount to 50 and click OK.
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